The present invention relates to a surface acoustic wave filter.
An ultrahigh-frequency surface acoustic wave filter comprises, in the simplest embodiment of this kind of filter, an input transducer electrically connected to the input terminal of the filter and an output transducer which is placed on the same piezo-electric substrate as this input transducer so as to be acoustically connected directly (that is to say without any obstacle) to the input transducer and which is electrically connected to the output terminal of the filter.
If it is desired to improve the specifications of such a filter, it proves necessary to increase the length of the substrate (that is to say to increase the distance which corresponds to the direct acoustic path between these two transducers) so as to extend the duration of its impulse response. In fact it is well known that a surface acoustic wave band filter has a response which is better when its impulse response is longer (the ideal filter is that which would have a impulse response of infinite duration). However, this conventional solution has the drawback of excessively increasing the size of this filter and consequently of burdening its manufacturing cost more than is desirable.
There is known, from document "Patent Abstracts of Japan, volume 9, No. 318(E-366) [2041], 13 Dec. 1985 and JP-A, 60-150312 (TOSHIBA), 8 Aug. 1985", a surface acoustic wave filter constituted according to a two-channel structure, each channel comprising respectively an input transducer electrically connected up to the input terminal of the filter and at least one output transducer electrically connected to the output terminal, the input transducer being acoustically connected directly to the output transducer, the output transducers being offset one with respect to the other by (n+1/4).lambda., where .lambda. is the wavelength of the wave propagating on the filter. There is no reflector (but another output transducer) connected acoustically to the input transducer of each of the channels.
There is also known, from U.S. Pat. No. 3,622,293, a surface acoustic wave device comprising an input transducer, an output transducer and a reflector (20) offset by .lambda./4 with respect to the output transducer.
The object of these two devices is only to eliminate the parasitic reflections, especially the triple-transit echo. Neither presents the problem of extending the duration of the impulse response.